Does A106 Gr.B pipe below 25 mm wall thickness require preheat under ASME B31.3?

Per ASME B31.3 Table 330.1.1, P-No.1 with wall thickness under 25 mm requires a minimum preheat of 10°C — essentially confirming the surface is free of moisture and not below 10°C. In Vietnam's climate (ambient temperature typically ≥ 25°C), this condition is usually met naturally, but must still be documented in the QA record. Some stricter WPS documents require 50–79°C even for thin-wall pipe to ensure complete moisture removal.

When welding P-No.1 pipe to a P-No.1 flange of different thickness, which thickness governs preheat?

The greater of the two thicknesses at the weld joint governs — not the average. This conservative rule ensures the thicker component, which cools more slowly and is more prone to residual stress accumulation, receives adequate heating. For Schedule 80 pipe (10.97 mm) welded to a Class 600 DN 6-inch flange hub (~38 mm), preheat is determined by the 38 mm flange hub dimension.

Does ASME B31.3 require preheat to be documented in the QA record?

ASME B31.3 does not prescribe a specific preheat record format, but requires WPS compliance and the Owner's Inspector has the right to request evidence. In practice on O&G and EPC projects in Vietnam, a Preheat Record (usually in ITR format — Inspection Test Record) is mandatory, documenting: time, temperature measured, measurement method (Tempilstik® part number), measurement location, and inspector name.

ASME B31.3 Preheat for Process Piping — P-Number Classification, Practical Guide & Tempilstik® Selection

Published 24 February 2025 · Fast Group Engineering · 9 min read

ASME B31.3 Process Piping governs the majority of piping installation work in refineries, gas plants, chemical plants, and industrial processing facilities in Vietnam. Unlike AWS D1.1 — which classifies steel by Carbon Equivalent and wall thickness — ASME B31.3 classifies materials by P-Number, ASME's own grouping system based on chemical composition and mechanical properties. Understanding P-Number and its corresponding preheat requirements is a fundamental competency for welding engineers, welding inspectors, and QA/QC personnel in process piping work.

Measuring preheat temperature on process piping using Tempilstik temperature indicating crayon at an industrial construction site — Each P-Number in ASME B31.3 carries different preheat requirements. Selecting the correct Tempilstik® part number by P-Number and wall thickness is essential for code-compliant preheat verification.

ASME B31.1 vs ASME B31.3 — Key Differences

These two codes are frequently confused, especially on projects that combine power generation with industrial processing systems:

Criterion	ASME B31.1 — Power Piping	ASME B31.3 — Process Piping
Scope	Power plants: main steam lines, steam extraction, boiler feedwater	Refineries, chemical plants, gas processing, LNG, industrial process systems
Inspection authority	Authorized Inspector (AI) per B31.1	Owner's Inspector per B31.3
PWHT requirements	More stringent — mandatory PWHT at lower wall thicknesses	Higher thickness thresholds before PWHT is triggered
Typical applications in Vietnam	Steam turbine piping, power plant boiler systems	BSR (Dung Quat Refinery), PVFCCo, GPP gas processing plants
P-Number system	Identical — governed by ASME Section IX

P-Number System in ASME B31.3

The P-Number is ASME Section IX's material grouping number — the foundation for developing WPS documents and determining preheat and PWHT requirements. The following table covers the P-Numbers most commonly encountered in process piping in Vietnam:

P-Number	Material Group	Typical Grade	Min Preheat (B31.3)	Tempilstik® Part No.
P-No.1	Carbon steel	A106 Gr.B, A53 Gr.B, API 5L Gr.B	10°C (t < 25 mm) / 79°C (t ≥ 25 mm)	#28009 (175°F / 79°C)
P-No.1 (heavier section)	Higher-carbon grade	A106 Gr.C, heavy wall	79°C / 121°C	#28009 / #28019
P-No.3	Low-alloy (0.5Mo)	A335 P1, A161 T1	79°C (175°F)	#28009
P-No.4	Low Cr-Mo (1.25Cr-0.5Mo)	A335 P11, A213 T11	150°C (302°F)	#28318
P-No.5A	Medium Cr-Mo (2.25–3Cr)	A335 P22, A335 P21	200°C (392°F)	#28327
P-No.5B	High Cr-Mo incl. P91 (5–9Cr)	A335 P91, A335 P9	200°C (392°F)	#28327
P-No.8	Austenitic stainless steel	A312 TP304, TP316, TP321	No preheat required	—

Note: The values above are reference figures from ASME B31.3 Table 330.1.1. Actual preheat requirements depend on the specific wall thickness, chemical composition from the Mill Test Certificate, and welding conditions. Always verify against the code original and the approved WPS. For a comprehensive lookup table see the preheat temperature reference table.

Case Study: Welding A106 Gr.B 6-inch Schedule 80 Pipe to a Class 600 Flange

This is one of the most common field situations in process piping — welding a pipe to a flange that is substantially thicker than the pipe wall. Preheat determination is more complex because the two components have significantly different wall thicknesses.

Component specifications

Component	Material	Standard	Thickness at weld
Pipe	ASTM A106 Gr.B — DN 6" (168.3 mm OD) Sch.80	P-No.1	10.97 mm
Flange	ASTM A105 — Class 600 — DN 6" (hub at weld bevel)	P-No.1	~38 mm

Step-by-step preheat determination

Confirm P-Number: Both A106 Gr.B (pipe) and A105 (flange) are P-No.1 (carbon steel). WPS: P-No.1 to P-No.1.
Determine governing thickness: Per ASME B31.3, when the two components have different thicknesses, preheat is determined by the greater thickness at the weld joint. The Class 600 DN 6" flange hub at the bevel is approximately 38 mm.
Apply Table 330.1.1: P-No.1 with t ≥ 25 mm → minimum preheat 79°C (175°F).
Select Tempilstik®: Part #28009 (175°F / 79°C).

Field measurement sequence

Heat the full pipe circumference and flange hub using resistance heating wire or propane torch.
Mark Tempilstik® #28009 at positions at least 75 mm from the weld bevel on the pipe side — at 4 evenly distributed points (0°, 90°, 180°, 270°).
Also mark the flange hub face — the thickest point, farthest from the heat source, slowest to reach temperature.
Begin welding only when all measurement points show the crayon has melted — confirming uniform preheat across the full joint.
Document in the Preheat Record (ITR): time, temperature, measurement method (Tempilstik® #28009), measurement locations, inspector name.

Interpass temperature for P-No.1: With P-No.1 carbon steel, maximum interpass temperature per most WPS is 250–300°C. If the WPS specifies interpass ≤ 250°C, use Part #28039 (450°F / 232°C) as the interpass check — when #28039 has not melted, the surface is below 232°C and safely within the 250°C limit.

Tempilstik crayon being applied to pipe surface to verify preheat temperature before welding per ASME B31.3 — Applying Tempilstik® to confirm preheat on a pipe-to-flange weld — mark at the flange hub as well as the pipe, since the hub is the thickest section and the last to reach temperature.

PWHT Requirements per ASME B31.3

ASME B31.3 defines wall thickness thresholds above which PWHT becomes mandatory for each P-Number. This is the second most important parameter after preheat:

P-Number	Wall Thickness Requiring PWHT (B31.3)	PWHT Temperature	Tempilstik® Spot-Check
P-No.1	t > 19 mm (¾")	595–650°C (1103–1202°F)	#28047 (316°C) — heating phase checkpoint
P-No.4	All thicknesses	675–760°C (1247–1400°F)	#28057 (550°C)
P-No.5A	All thicknesses	675–760°C (1247–1400°F)	#28057 (550°C)
P-No.5B (P91)	All thicknesses — no exceptions	730–760°C (1346–1400°F)	#28065 (788°C)
P-No.8	Not applicable	—	—

For full PWHT procedure details, soaking time requirements, and the role of Tempilstik® in heat treatment verification, see PWHT — Post Weld Heat Treatment.

Frequently Asked Questions

Per ASME B31.3 Table 330.1.1, P-No.1 with wall thickness under 25 mm requires a minimum preheat of 10°C — in practice, confirming the surface is free of moisture and above 10°C. In Vietnam's climate, this is generally met naturally, but must still be documented. Some stricter WPS documents require 50–79°C even for thin-wall pipe to fully eliminate moisture.

The greater of the two thicknesses at the weld joint governs — not an average. This conservative rule ensures the thicker component, which is more prone to residual stress accumulation, receives adequate preheat. For Sch.80 pipe (10.97 mm) welded to a Class 600 flange hub (~38 mm), preheat is governed by the 38 mm hub dimension.

Temperature at the bevel face reflects the instantaneous surface temperature — not the through-thickness heat penetration. Measuring 75 mm from the weld edge confirms that the metal which will become the HAZ has also reached the required temperature, not just the surface exposed directly to the heat source.

ASME B31.3 does not prescribe a specific preheat record format, but the Owner's Inspector has the right to request evidence of WPS compliance. In practice on O&G and EPC projects in Vietnam, a Preheat Record in ITR format is mandatory — documenting time, temperature measured, measurement method (Tempilstik® part number), measurement location, and inspector name.

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Preheat Temperature Reference Table

AWS D1.1, ASME B31.3, API 1104 — full lookup with part numbers.

PWHT — Post Weld Heat Treatment

ASME requirements by P-Number and Tempilstik® PWHT spot-check role.

Interpass Temperature Control — AWS & ASME

Maximum limits by material and field measurement procedure.